Improved thermal stability and oxidation resistance of Al-Ti-N coating by Si addition

Addition of Si is very effective in upgrading the machining performance and thermal properties of Al-Ti-N coating. Here, we concentrate on the thermal stability and oxidation resistance of Al-Ti-Si-N coating. Alloying with Si favors the growth of wurtzite phase, and thereby causes a drop in hardness from similar to 34.5 to 28.7 GPa. However, Si-containing coating retards the formation of w-AlN during thermal annealing, and thereby behaves a high hardness value of similar to 31.3 GPa after annealing at T-a = 1100 degrees C. After 10 h exposure in air at 850 degrees C, Al-Ti-N coating is fully oxidized. Incorporation of Si significantly improves the oxidation resistance of Al-Ti-N due to the combined effects with the promoted formation of Al-oxide rich top-scale and retarded transformation of anatase (a-) TiO2 into rutile (r-) TiO2, where only similar to 1.43 mu m oxide scale is shown after oxidation at 1100 degrees C for 15 h. Noticeable is that the worst oxidation resistance of Al-Ti-Si-N coating in the temperature range from 800 to 1100 degrees C is obtained at 950 degrees C with oxide scale of similar to 1.76 mu m due to the fast formation of r-TiO2. Additionally, a pre-oxidation at 1000 degrees C has a positive effect on the oxidation resistance of Al-Ti-Si-N coating, which is attributed to the formation of Al-oxide rich top-scale, and thus inhibits the outward diffusion of metal atoms and inward diffusion of O. (C) 2014 Elsevier B. V. All rights reserved.